Production method of warm- or hot-formed product

ABSTRACT

Disclosed is a method wherein when a steel sheet is subjected to warm or hot forming to produce a formed product by drawing with a punch and a die, the steel sheet is formed while the forming start temperature is controlled in accordance with the heating temperature of the steel sheet. By this method, when a steel sheet is subjected to warm or hot forming, good formability is obtained without the occurrence of fracture, cracking or the like during forming and also a formed product having good ductility is obtained.

BACKGROUND OF THE INVENTION

The present invention relates to, in the field of producing a steel sheet formed product that is mainly applied to an automobile body: a method for producing a formed product by heating a steel sheet (blank) as the raw material to a temperature of not lower than the austenite and ferrite formation temperature (Ac1 transformation temperature) and subjecting the heated steel sheet to press forming; and a formed product obtained by the production method. In particular, the present invention relates to: a method for producing a formed product that assures good formability without causing fracture, cracking, etc. during press forming; and a formed product thereof.

In the field of automobile parts, the strengthening of the materials for the parts is promoted with the aim of securing both collision safety performance and weight reduction at the same time. Those parts are generally produced by press-forming a steel sheet. However, when cold forming is applied to a highly strengthened steel sheet, the material is hardly formed particularly if it exceeds 980 MPa.

In view of the above situation, hot forming technologies of forming a steel sheet material in a heated state have been studied. As one of the technologies, for example, JP-A No. 102980/2002 proposes the technology of forming a metal material in the state of heating it to 850° C. to 1,050° C. with a press tool set of a relatively low temperature. It is said that the technology improves the formability of a metal material and also prevents delayed fracture caused by residual stress from occurring. When a high-strength steel sheet of 1,470 MPa class in tensile strength that has been considered to be hardly formable by a conventional cold forming method is used as a material in particular, the technology makes it possible to obtain a part having a relevant strength and good dimensional accuracy.

FIG. 1 is an explanatory schematic view showing a tool set configuration to apply such hot forming (hereunder referred to as “hot stamping” occasionally) as described above. In the figure, the reference numeral 1 shows a punch, 2 a die, 3 a blank holder, 4 a steel sheet (material), the reference character BHF a blank holding force, rp a punch shoulder radius, rd a die shoulder radius, and CL the clearance between the punch and the die. Here, among those forming parts, the punch 1 and the die 2 are so configured that passages 1 a and 2 a, through which a coolant (for example, water) can pass, are formed therein, respectively, and the members can be cooled by passing the coolant through the passages.

When hot stamping (for example, hot deep drawing) is applied with such a tool set, the forming is started while a blank (a steel sheet 4) is heated to a temperature not lower than the Ac3 transformation temperature and softened. That is, the steel sheet 4 is pushed into the hole of the die 2 with the punch 1 while the steel sheet 4 of a high temperature is held between the die 2 and the blank holder 3, the outer diameter of the steel sheet 4 is reduced, and in the meantime the steel sheet 4 is formed into a shape conforming to the outer shape of the punch 1. Meanwhile, by cooling the punch and die in parallel with the forming, heat of the steel sheet 4 is transferred to the tool set (the punch and die), and by further cooling and keeping it at the bottom dead center in forming, the material is hardened. By applying such a forming method, a part of 1,470 MPa class having good dimensional accuracy can be obtained and moreover, in comparison with the case of cold forming a part of the same strength level, the load required for the forming can be reduced and thus the capacity of a press machine can be reduced.

However, since the timing when a heated blank touches a tool set varies by the sites of the heated blank, temperature difference appears in the blank and thus unevenness of the material strength caused by the temperature difference is likely to appear in the blank. In the case of deep drawing that requires a blank holder in particular, the temperature at the flange portion of the blank held between the blank holder and the die lowers rapidly during forming. Since the flow stress of the material increases with such temperature drop, the material tends to fracture during forming. Therefore, the problem has been that, even when a blank is heated and softened intentionally, deep drawing can not be applied because of the above reasons.

Further, in conventional hot forming, since a blank is once heated to a temperature not lower than the Ac3 transformation temperature, the microstructure of the formed product becomes mostly composed of a martensite structure due to the rapid cooling caused by a tool set after the forming. As a result, a part having an ultra-high strength of 1,470 MPa or more can be obtained but, since the microstructure of the part is composed of martensite, the ductility of the part is inferior. This means that the part may have the possibility of fracture in some circumstances, for example, when an automobile collision occurs and the part is deformed. When the part fractures, the part cannot absorb collision force at the moment and resultantly the damage to a passenger may possibly increase. For those reasons, it cannot be said that a part formed by hot stamping always has a wider range of application and it is the present situation that the advantages of both high strength and good dimensional accuracy cannot be utilized enough.

SUMMARY OF THE INVENTION

The present invention has been established in view of the above situation and the object thereof is to provide: a method for producing a formed product by hot stamping, the formed product having a wider application range by securing good formability and good ductility without the occurrence of fracture and cracking during forming when a steel sheet is subjected to hot or cold forming; and the formed product that can exhibit such advantages.

One aspect of a production method of a warm- or hot-formed product which has attained the above object according to the present invention comprises the steps of: heating a steel sheet to a temperature not lower than the Ac1 transformation temperature; cooling the steel sheet to a temperature in the range from higher than the martensite transformation start temperature Ms point of the steel sheet to lower than the temperature determined in accordance with the heating temperature; and forming the cooled steel sheet with a punch and a die.

In the aspect of the production method, it is preferable that: the steel sheet is heated to a temperature in the range from not lower than the Ac1 transformation temperature to lower than the Ac3 transformation temperature in the heating step; and the temperature determined in accordance with the heating temperature satisfies the following expression (1): Forming start temperature (° C.)<0.725×Heating temperature of a steel sheet (° C.)   (1).

Otherwise, in the aspect of the production method, it is preferable that: the steel sheet is heated to a temperature not lower than the Ac3 transformation temperature in the heating step; and the temperature determined in accordance with the heating temperature is 600° C.

Further, in the aspect of the production method, it is preferable that the step of forming the cooled steel sheet with a punch and a die is finished during the time when the temperature of the steel sheet is higher than the temperature Ms point.

Further, in the aspect of the production method, a blank holder may be used when the steel sheet is formed in the step of forming the cooled steel sheet with the punch and the die.

By the aspect of the present invention, since the forming start temperature is made to be controllable in accordance with the heating temperature of a steel sheet when the steel sheet is hot formed or warm formed, good formability can be secured without the occurrence of fracture and cracking during forming, thus a formed product that shows good ductility can be produced, and thereby the application range of the steel sheet is expected to expand.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory schematic view showing a tool set configuration for hot forming.

FIG. 2 is an explanatory schematic view showing a tool set configuration developed previously.

FIG. 3 is a graph showing the influence of a forming start temperature and a heating temperature on a flow stress.

FIG. 4 is a graph showing the relationship between the forming temperature at which a fracture stress exceeds a flow stress and a heating temperature.

FIG. 5 is a perspective view illustratively showing the appearance of a successfully formed product.

FIG. 6 is a graph showing the relationship between the cooling start temperature and the Vickers hardness (at a load of 9.8N) of a formed product.

FIG. 7 is a graph produced by putting in order the tensile strength and total elongation of a formed product in relation to a ferrite fraction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have heretofore been studying a technology that can realize good formability and as a part of the study have proposed the technology of deep drawing with the tool set shown in FIG. 2. In this tool set configuration, pins 7 to support a steel sheet are disposed at portions of a blank holder 3 and the steel sheet can keep close to a die 2 and the blank holder 3 without directly touching them by placing the steel sheet 4 on the pins 7, (the other part of the configuration in FIG. 2 is basically identical to FIG. 1). Then, it is configured so that, at the time of forming, the upper faces of the pins 7 are on the same plane as the upper face of the blank holder and the steel sheet 4 is in the state of being mounted on the blank holder 3.

In such a tool set configuration, the steel sheet 4 is supported by the pins 7, thus direct contact between the steel sheet 4 and the tool set (particularly the die 2 and the blank holder 3) can be avoided before forming, thereby the portion of the steel sheet 4 above the upper face of the punch 1 and most of the other portions thereof are cooled nearly simultaneously. Hence the disadvantage that the material strength of the steel sheet 4 at the punch face lowers in comparison with the material strength thereof at the flange face due to the unevenness in the temperature of the steel sheet 4 can be prevented. As a result, fracture at the punch face is prevented in particular and drawability can be improved.

By those technologies, the drawability of a steel sheet has improved dramatically but it has been found that the ductility of a formed product is not improved yet in some cases. That is, by the above proposed technologies or the technology proposed by the present inventors, the structure of the formed product is mainly composed of martensite due to the forming start temperature, forming temperature, forming termination temperature and the like. It has been estimated that this is the reason why the good ductility of a formed product cannot be maintained.

In this light, the present inventors have studied from various viewpoints in order to solve the drawback. As a result, the present inventors have found that the above object can be excellently attained by controlling the forming start temperature in accordance with the heating temperature of a steel sheet and have established the present invention. The present invention is hereunder explained concretely along with the steps to the establishment of the present invention.

The present inventors firstly heated the steel sheet having the chemical composition shown in Table 1 below to 900° C. (the Ac1 and Ac3 transformation temperatures of the steel sheet were 725° C. and 850° C., respectively), and subjected the steel sheet to the deep drawing test through the aforementioned procedure with the tool set shown in FIG. 2. As a result, the present inventors confirmed that, when the steel sheet was formed in the state wherein the temperature of the blank lowered though it took time from the heating to the start of forming, the blank which had cracked till then during forming did not crack and could be formed. From this result, it was estimated that if a blank once heated was cooled intentionally and then the forming of the blank was started, the drawability improved, though it had historically been thought to be a technological common sense to start forming at a temperature as high as possible in the case of hot forming. TABLE 1 Chemical composition of blank (mass %) C Si Mn Cr B Ti P S Balance 0.2 0.19 1.22 0.34 0.0019 0.020 0.010 0.010 Fe

In this light, the mechanism was further studied and as a result the present inventors came to the thought that such a phenomenon resulted from the fact that the balance (magnitude relation) between the stress required for feeding a blank into the interior of a die (the stress being hereunder referred to as “flow stress” occasionally) while drawing (compressing) the flange portion in a deep drawing step and the fracture stress at the punch shoulder portion and the vertical wall portion where the material undergoing the flow stress flew into the interior of the die (the stress being hereunder referred to as “fracture stress” occasionally) varied in accordance with the forming temperature.

The present inventors produced compression test pieces of a columnar shape separately, once heated them to 700° C., 800° C. and 900° C., thereafter cooled them to 500° C., 600° C., 700° C. and 800° C. at a cooling rate of 20° C./sec., and measured the average 10%-deformation stress (corresponding to the flow stress required for the drawing of a flange portion) when they were subjected to the compression test while they were maintained at the relevant temperatures. Further, the present inventors carried out similar tests using tensile test pieces and measured the fracture stress (corresponding to the “fracture stress” at the punch shoulder portion and the vertical wall portion). The results are shown in FIG. 3 (graph showing the influence of a forming start temperature and a heating temperature on a flow stress) and the region wherein the fracture stress at a punch shoulder portion and a vertical wall portion exceeded the flow stress at the flange portion was clearly identified and it was clarified that the relationship varied in accordance to the heating temperature (refer to the example to be described later).

The relationship between a forming temperature at which a fracture stress exceeds a flow stress and a heating temperature, which was obtained on the basis of the above results, is shown in FIG. 4. In FIG. 4, the marks “◯” mean the cases where cracking or the like did not occur, good formability was obtained, and moreover the ductility of the formed products was also good, the marks “x” the cases where fracture or the like occurred, and the mark “Δ” the case where good formability was obtained but the ductility of the formed product deteriorated. It is obvious from the results that, if a forming start temperature is controlled in accordance with the heating temperature of a steel sheet, good formability can be obtained and also the ductility of the formed product improves. An example of the appearance of a successfully formed product is shown in FIG. 5 (schematic view). Next, concrete conditions stipulated in the present invention are explained.

As shown in FIG. 4, the region where fracture occurs can obviously be distinguished from the region where good formability (and ductility) is obtained. As a result of putting in order and studying that relation, when a heating temperature is in the range from not lower than the Ac1 transformation temperature (725° C.) to lower than the Ac3 transformation temperature (850° C.), good formability can be obtained and also the ductility of the formed product is good as long as the aforementioned expression (1) is satisfied. Further, in a product formed under such conditions, ferrite is already formed in some portions of the microstructure of the blank in the heating step and the ferrite fraction in this case is 10% or more in area percentage.

Meanwhile, it has also been clarified that, when a blank is heated to a temperature higher than the Ac3 transformation temperature, in order to have the microstructure of the formed product not mainly composed of martensite, introduce ferrite actively, and thus improve the ductility of the formed product, it is only necessary to control the forming start temperature to a temperature lower than 600° C. When the forming start temperature is 600° C. or higher in that case, an austenite single phase structure is still maintained even at the time of the completion of the forming (at the time when a tool set reaches the bottom dead center), the microstructure is transformed into a structure mainly composed of martensite by the hardening caused by the heat dissipation of the tool set at the bottom dead center, and thus a formed product having good ductility is not obtained (the mark “A” in FIG. 4). The phenomenon has been clarified from the experiments wherein the hardening with a tool set is simulated by heating a steel sheet to 900° C., thereafter cooling it to various temperatures, and holding it between thick steel sheets. The relationship between the cooling (rapid cooling) start temperature and the Vickers hardness (at a load of 9.8N) of the formed product in this case is shown in FIG. 6. From the figure, it is understood that, by controlling a cooling start temperature to lower than 600° C., the formation of ferrite is accelerated and the hardness of the steel sheet lowers. Here, in this case, the average cooling rate was 10 to 20° C./sec. in the temperature range from the heating temperature to the temperature at the time of holding the steel sheet in between (hardening temperature). Even by applying such production conditions, it becomes possible to actively introduce ferrite into the microstructure of a formed product, the ferrite fraction becomes 10% or more in area percentage, and good ductility is obtained. Here, the hardness was measured in the vicinity of the center of the sheet thickness at a center portion of the vertical wall of the formed product (FIG. 3).

Note that, when the heating temperature of a blank is set at the Ac3 transformation temperature or higher, it is preferable that the upper limit thereof is about 1,000° C. at the highest. If the temperature exceeds 1,000° C., it is concerned that oxided scale forms abundantly (for example 100 μm or more) and the formed product (after subjected to descaling) becomes thinner than the prescribed thickness.

Whatever heating temperature may be adopted, it is necessary that the lower limit of a forming start temperature is a temperature higher than the martensite transformation start temperature Ms point (refer to FIG. 4). If a forming start temperature is lower than the martensite transformation start temperature, martensite transformation undesirably occurs during forming (before a tool set reaches the bottom dead center in forming) and the forming can hardly be continued at the moment. In the present invention, as far as a forming start temperature is controlled in relation to a heating temperature, the above object can be attained. With regard to a forming termination temperature, though the temperature is not particularly limited, from the viewpoint of reducing the amount of the martensite structure appearing during forming as much as possible, it is preferable that the forming termination temperature is also a temperature higher than the martensite transformation start temperature. Further, as a preferable embodiment, it is preferable that the time duration from the start of forming (when a blank touches a part of a tool set except pins 7 shown in FIG. 2) to the termination of the forming is within two seconds, and by adding this condition, fracture is prevented more reliably during forming.

According to the method of the present invention, the aforementioned object can be attained by properly controlling the relationship between a heating temperature and a forming start temperature. Those effects can conspicuously be exhibited when a steel sheet is formed with a tool set equipped with a blank holder (namely deep drawing) and, in addition to this requirement, it is also effective to use the technologies proposed earlier in combination. That is, it is also effective to equalize the temperature of a steel sheet by employing the die configuration shown in FIG. 2 or to subject a steel sheet having oxided scale 15 μm or more in thickness on the surface to press forming and, by using those technologies in combination, the effects of the present invention can be exhibited more effectively. Note that, even when a steel sheet is formed while those configurations are added, the aforementioned production conditions stipulated in the present invention are not changed basically.

As it is obvious from the above tenor, a formed product according to the present invention is not limited to the formed product drawn by using a blank holder but includes a formed product obtained through ordinary press forming. Even in the case of producing such a formed product obtained through ordinary press, the effects of the present invention can be attained.

Here, the hot region cited in the present invention means the temperature region of the recrystallization temperature or higher and the warm region means the temperature region from the ordinary temperature to the recrystallization temperature.

The method according to the present invention is applicable to a steel sheet having a chemical composition of a very wide range. Basically, as far as a steel has hardenability, namely a steel contains C by 0.1% or more, the method is applicable to the steel.

The effects of the present invention are hereunder explained more specifically on the basis of the examples but the examples do not limit the present invention and any design change conforming to the tenor of the present invention is included in the technological scope of the present invention.

[Example]

A steel having the chemical composition shown in Table 1 was rolled to a thickness of 1.4 mm and annealed by ordinary means. Round blanks 95 mm in diameter (blank diameter) were stamped from the rolled steel sheet and used for tests (the Ac1 and Ac3 transformation temperatures of the blanks were accordingly 725° C. and 850° C., respectively).

The round blanks were subjected to square-shell drawing while it was warm or hot with a tool set having a square-shaped punch head (the tool set comprising a rectangular die and a rectangular punch and the length of each side being 45 mm, refer to FIG. 2) by the method of the present invention. In this case, the blanks were heated in an atmospheric air in an electric furnace and the heating temperature was changed variously. Further, by controlling the heat retention time for each heating temperature at the time of heating, the thickness of the oxided scale formed during heating was equalized to be about 20 μm.

The forming test was carried out with the tool set, shown in FIG. 2, incorporated into a crank press machine. The time duration from the time when the tool set touched the blank to the time when the tool set stopped at the bottom dead center was set at 0.75 second. Further, the forming start temperature was controlled by controlling the cooling time duration from the time when the blank was taken out from a heating furnace to the time when the forming was started, and at the same time the actual temperatures were measured with a radiation thermometer. The average cooling rate at the time was set at 10 to 20° C./sec. in the range from the heating temperature to the forming start temperature. In the forming step, the blanks were held for about 20 sec. after the start of the forming at the bottom dead center and then subjected to hardening. Other press forming conditions were as follows:

-   -   (Other press forming conditions)     -   Blank holding force: 1 ton,     -   Die shoulder radius rd: 5 mm,     -   Punch shoulder radius rp: 5 mm,     -   Clearance between punch and die CL: (1.32/2+1.4 (steel sheet         thickness)] mm,     -   Forming height: 37 mm, and     -   Lubricant: A solid lubricant, in the state of paste, the         allowable temperature limit of which was 1,000° C. was applied         to the tool set.

After the forming, the hardness at a section, the microstructure and the ferrite fraction of the formed product were measured. With regard to the measurement of the ductility of the formed product, since it was difficult to cut out a tensile test piece from the formed product, a steel sheet was prepared so as to simulate hardening at the bottom dead center of forming by heating the same steel sheet as used in the forming test, thereafter cooling it naturally to the forming start temperature, and right after that holding it between plate steels 10 mm in thickness, and a JIS #13B test piece was cut out from the simulated steel sheet and subjected to the tensile test and the measurement of the total elongation. The hardness (Vickers hardness Hv, 9.8N load) was measured in the vicinity of the center of the sheet thickness at a center portion of the vertical wall of the formed product (FIG. 5). Further, the formability was judged by the occurrence of fracture and shown by the mark “◯” in the case of no fracture and the mark “x” in the case of fracture.

Those results are correctively shown in Table 2 below together with the production conditions. Further, FIG. 7 shows the graph produced by putting in order the tensile strength and total elongation in relation to the ferrite fraction on the basis of the results. Here, FIG. 4 is the graph produced by putting the data in order on the basis of the same results. TABLE 2 Heating Forming start tempera- tempera- Ferrite Tensile Total ture ture Vickers fraction strength elongation NO. (° C.) (° C.) Drawability hardness (Hv) Microstructure (area %) (MPa) (%) Remarks 1 900 780 X — — less than 10% 1470 2 Fracture 2 900 645 ◯ 463 M less than 10% 1381 3 Micro-cracking 3 900 595 ◯ 339 M + F 25 1009 10 — 4 900 700 ◯ 485 M less than 10% 1489 1 Micro-cracking 5 900 550 ◯ 330 M + F 30 980 11 — 6 860 580 ◯ 340 M + F 27 1009 12 — 7 860 690 X — — less than 10% 1421 2 Fracture 8 800 550 ◯ 325 M + F 25 980 8 — 9 800 575 ◯ 358 M + F 27 1087 10 — 10 800 600 X — — — 1107 7 Fracture 11 750 500 ◯ 280 M + F 45 813 18 — 12 750 520 ◯ 265 M + F 40 803 17 — 13 750 500 ◯ 276 M + F 55 784 19 — 14 750 590 X — — — 735 20 Fracture 15 800 640 ◯ 380 M + F 15 1078 15 — 16 950 550 ◯ 378 M + F 17 1234 8 — 17 950 500 ◯ 345 M + F 20 1127 7 — M: Martensite F: Ferrite

As it is obvious from the results, when a steel sheet is formed under the conditions stipulated in the present invention, good formability is obtained and also a formed product having good ductility is obtained.

The foregoing invention has been described in terms of preferred embodiments. However, those skilled, in the art will recognize that many variations of such embodiments exist. Such variations are intended to be within the scope of the present invention and the appended claims. 

1. A production method of a warm- or hot-formed product, said production method comprising the steps of: heating a steel sheet to a temperature not lower than the Ac1 transformation temperature; cooling said steel sheet to a temperature in the range from higher than the martensite transformation start temperature Ms point of said steel sheet to lower than the temperature determined in accordance with said heating temperature; and forming the cooled steel sheet with a punch and a die.
 2. The production method according to claim 1, wherein: said steel sheet is heated to a temperature in the range from not lower than the Ac1 transformation temperature to lower than the Ac3 transformation temperature in said heating step; and said temperature determined in accordance with said heating temperature satisfies the following expression (1): Forming start temperature (° C.)≦0.725×Heating temperature of a steel sheet (° C.)   (1).
 3. The production method according to claim 1, wherein: said steel sheet is heated to a temperature not lower than the Ac3 transformation temperature in said heating step; and said temperature determined in accordance with said heating temperature is 600° C.
 4. The production method according to claim 1, wherein said step of forming the cooled steel sheet with the punch and the die is completed during the time when the temperature of said steel sheet is higher than said temperature Ms point.
 5. The production method according to claim 1, wherein a blank holder is used when said steel sheet is formed in said step of forming the cooled steel sheet with the punch and the die. 